Sediment-laden fresh water above salt water: nonlinear simulations
Sediment-laden fresh water above salt water: nonlinear simulations
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DOI：
10.1017/jfm.2014.645
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发表时间：
2015-01-01
影响因子：
3.7
通讯作者：
Meiburg, E.
中科院分区：
文献类型：
--
作者：
Burns, P.;Meiburg, E.
当一层充满颗粒的淡水被置于清澈的盐水之上时，双扩散和瑞利-泰勒不稳定性都可能出现。本研究扩展了Burns & Meiburg（J. Fluid Mech.，第691卷，2012年，pp. 279-314）到非线性制度，通过二维和三维直接数值模拟（DNS）。DNS中的初始不稳定性增长被认为是一致的线性稳定性分析预测的主导模式。随后，个别手指的蓬勃发展引起了二次不稳定，并最终形成强烈的羽流，成为脱离界面区域。模拟结果表明，斯托克斯沉降速度的颗粒的存在下，通过创建一个不稳定的“鼻区”的水平平均配置文件，位于向上移动的盐度和向下移动的沉积物界面之间，修改了传统的双扩散指。盐度（沉积物）界面的有效厚度l（s）（l（c））扩散性地增长，鼻状区域的高度H也是如此。H/l（s）的比值最初增大，然后趋于稳定，其值由从上方进入玫瑰区的沉积物通量、从下方鼻区流出的双扩散/瑞利-泰勒通量以及鼻区内沉积物累积速率之间的平衡决定。对于小的H/l（s）= O（0.1）的值，沉积物和盐度界面在空间上变得越来越分离，并且主导的不稳定模式变得像Rayleigh-Taylor。基于参数研究结果的比例分析表明，H/l（s）是一个单一无量纲分组的线性函数，可以解释为流入鼻状区的沉积物流入和流出的比率。模拟结果还表明，双扩散和瑞利-泰勒不稳定机制导致沉积物的有效沉降速度与系统的总体浮力速度成比例，其可以比斯托克斯沉降速度大几个数量级。虽然双扩散流和瑞利-泰勒主导流的功率谱在性质上是相似的，但在分析谱相移时，可以清楚地看到指进流和泄漏流之间的差异。对于泄漏占主导地位的流量的锁相机制，观察到，这加剧了随着时间的推移。因此，泄漏模态可以解释为指进模态，由于在机头区域的大尺度翻转事件，由于瑞利-泰勒不稳定性，指进模态已经锁相。
When a layer of particle-laden fresh water is placed above clear, saline water, both double-diffusive and Rayleigh-Taylor instabilities may arise. The present investigation extends the linear stability analysis of Burns & Meiburg (J. Fluid Mech., vol. 691, 2012, pp. 279-314) into the nonlinear regime, by means of two-and three-dimensional direct numerical simulations (DNS). The initial instability growth in the DNS is seen to be consistent with the dominant modes predicted by the linear stability analysis. The subsequent vigorous growth of individual fingers gives rise to a secondary instability, and eventually to the formation of intense plumes that become detached from the interfacial region. The simulations show that the presence of particles with a Stokes settling velocity modifies the traditional double-diffusive fingering by creating an unstable 'nose region' in the horizontally averaged profiles, located between the upward-moving salinity and the downward-moving sediment interface. The effective thickness l(s) (l(c)) of the salinity (sediment) interface grows diffusively, as does the height H of the nose region. The ratio H/l(s) initially grows and then plateaus, at a value that is determined by the balance between the flux of sediment into the rose region from above, the double-diffusive/Rayleigh-Taylor flux out of the nose region below, and the rate of sediment accumulation within the nose region. For small values of H/l(s) = O(0.1) the sediment and salinity interfaces become increasingly separated in space and the dominant instability mode becomes Rayleigh-Taylor like. A scaling analysis based on the results of a parametric study indicates that H/l(s) is a linear function of a single dimensionless grouping that can be interpreted as the ratio of inflow and outflow of sediment into the nose region. The simulation results furthermore indicate that double-diffusive and Rayleigh-Taylor instability mechanisms cause the effective settling velocity of the sediment to scale with the overall buoyancy velocity of the system, which can be orders of magnitude larger than the Stokes settling velocity. While the power spectra of double-diffusive and Rayleigh-Taylor-dominated flows are qualitatively similar, the difference between flows dominated by fingering and leaking is clearly seen when analysing the spectral phase shift. For leaking-dominated flows a phase-locking mechanism is observed, which intensifies with time. Hence, the leaking mode can be interpreted as a fingering mode which has become phase-locked due to large-scale overturning events in the nose region, as a result of a Rayleigh-Taylor instability.